You can stop your plan yourself, and you keep what you've paid for until the period runs out. Go to Settings → Plan & billing and choose Cancel plan. On Before you go we ask what made you decide, because it decides what we build next. Depending on your reason we might offer a smaller plan, a discount or a month's pause first. The discount can only be taken once per account. Say no and you land on Cancel your plan?, where Cancel plan confirms it.
Your plan stays fully active until the end of the period you've already paid for; we don't refund the rest of it. After that the billing stops and there are no further charges, your Series stop making new uploads, and whatever monthly usage you had left goes with the period.
Nothing you've made goes away. The videos already on your YouTube channel stay up, the commercial rights to everything you made are yours, and your Library stays on your account. Usage packs you bought stay too, until they expire. Without a plan they still let you make and download videos. Publishing to YouTube, Series and agents that create or publish for you need a plan again.
Changed your mind before the period ends? Choose Resume plan at Settings → Plan & billing and everything carries on as it was. After it ends, pick a plan again and your Library is still there.
If you'd rather step away for a while than end things, Pausing your subscription has the shorter option.